netdev - Re: [RFC PATCH bpf-next 00/14] xdp

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Date:   Fri, 16 Aug 2019 08:59:11 -0700
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Toshiaki Makita <toshiaki.makita1@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        "David S. Miller" <davem@...emloft.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, William Tu <u9012063@...il.com>
Subject: Re: [RFC PATCH bpf-next 00/14] xdp_flow: Flow offload to XDP

On 08/15, Jakub Kicinski wrote:
> On Thu, 15 Aug 2019 08:21:00 -0700, Stanislav Fomichev wrote:
> > On 08/15, Toshiaki Makita wrote:
> > > On 2019/08/15 2:07, Stanislav Fomichev wrote:  
> > > > On 08/13, Toshiaki Makita wrote:  
> > > > > * Implementation
> > > > > 
> > > > > xdp_flow makes use of UMH to load an eBPF program for XDP, similar to
> > > > > bpfilter. The difference is that xdp_flow does not generate the eBPF
> > > > > program dynamically but a prebuilt program is embedded in UMH. This is
> > > > > mainly because flow insertion is considerably frequent. If we generate
> > > > > and load an eBPF program on each insertion of a flow, the latency of the
> > > > > first packet of ping in above test will incease, which I want to avoid.  
> > > > Can this be instead implemented with a new hook that will be called
> > > > for TC events? This hook can write to perf event buffer and control
> > > > plane will insert/remove/modify flow tables in the BPF maps (contol
> > > > plane will also install xdp program).
> > > > 
> > > > Why do we need UMH? What am I missing?  
> > > 
> > > So you suggest doing everything in xdp_flow kmod?  
> > You probably don't even need xdp_flow kmod. Add new tc "offload" mode
> > (bypass) that dumps every command via netlink (or calls the BPF hook
> > where you can dump it into perf event buffer) and then read that info
> > from userspace and install xdp programs and modify flow tables.
> > I don't think you need any kernel changes besides that stream
> > of data from the kernel about qdisc/tc flow creation/removal/etc.
> 
> There's a certain allure in bringing the in-kernel BPF translation
> infrastructure forward. OTOH from system architecture perspective IMHO
> it does seem like a task best handed in user space. bpfilter can replace
> iptables completely, here we're looking at an acceleration relatively
> loosely coupled with flower.
Even for bpfilter I would've solved it using something similar:
iptables bypass + redirect iptables netlink requests to some
userspace helper that was registered to be iptables compatibility
manager. And then, again, it becomes a purely userspace problem.

The issue with UMH is that the helper has to be statically compiled
from the kernel tree, which means we can't bring in any dependencies
(stuff like libkefir you mentioned below).

But I digress :-)

> FWIW Quentin spent some time working on a universal flow rule to BPF
> translation library:
> 
> https://github.com/Netronome/libkefir
> 
> A lot remains to be done there, but flower front end is one of the
> targets. A library can be tuned for any application, without a
> dependency on flower uAPI.
> 
> > But, I haven't looked at the series deeply, so I might be missing
> > something :-)
> 
> I don't think you are :)